As the energy storage technique is extensively applied to cellular phones, camcorders, notebooks and electric vehicles, high energy density is demanded to the battery used as a power source of such an electric device. A lithium secondary battery is a battery capable of meeting such a requirement best and is being actively studied.
Lithium secondary batteries developed in the early 1990's are made up of a negative electrode of a carbon-based material capable of intercalating and deintercalating lithium ions, a positive electrode made of lithium-containing oxide, and a nonaqueous electrolyte containing a proper amount of lithium salts dissolved in a mixed organic solvent.
The average discharge voltage of the lithium secondary battery is about 3.6 to 3.7 V, which is higher than those of alkali batteries, nickel-cadmium batteries or the like. For such a high operating voltage, an electrolytic composition electrochemically stable in a charge/discharge range of 0 to 4.2 V is required. For this, a mixed solvent where a cyclic carbonate compound such as ethylene carbonate and propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate are appropriately mixed is used as a solvent of the electrolyte. A solute of the electrolyte commonly uses a lithium salt such as LiPF6, LiBF4, LiClO4 or the like, which serves as a lithium ion source in a battery and thus enables the lithium battery to operate.
In particular, in recent years, due to the tendency for a light, thin, short and small design of an electronic product and the increase of power consumption, a lithium secondary battery used as an energy source of such an electronic device has larger capacity. Large capacity of a lithium secondary battery may be obtained by using a high-capacity active material, increasing a capacity by raising a charging voltage or utilizing an inner space more efficiently. However, if a nickel-based ternary-system positive electrode material spotlighted in these days is used to raise a charging voltage, decomposition reaction may abruptly occur at the electrolyte, which may deteriorate battery performance such as life cycle. This phenomenon may become worse depending on service environment, and if the service temperature increases, the deterioration of battery performance becomes rapidly increasing. In addition, an electrode density tends to gradually increase in line with the tendency of maximizing utilization of a battery inner space, and this demands an electrolyte with low viscosity and high ion conductivity. However, an electrolyte having such characteristics generally has bad stability against oxidation reaction.